Regardless of the type of an internal combustion engine (for example reciprocating piston engine, pistonless rotary engine or free-piston engine), noises are generated as a result of the successively executed strokes (in particular intake and compression of the fuel-air mixture, combustion and discharge of the combusted fuel-air mixture). On the one hand, the noises propagate through the internal combustion engine in the form of solid-borne sound and are emitted on the outside of the internal combustion engine in the form of airborne sound. On the other hand, the noises propagate in the form of airborne sound together with the combusted fuel-air mixture through an exhaust system that is in fluid communication with the internal combustion engine. The noise propagating through the exhaust system in form of airborne sound is referred to as exhaust noise.
These noises are often regarded as being disadvantageous. On the one hand, there are statutory provisions on protection against noise to be observed by manufacturers of vehicles driven by internal combustion engines. These statutory provisions normally specify a maximum sound pressure for an operation of a vehicle. Manufacturers, on the other hand, try to impart a characteristic noise emission to internal combustion engine driven vehicles of their production, with the noise emission fitting the image of the respective manufacturer and being popular with customers. Present-day engines with small displacement often cannot naturally generate such intended characteristic noise.
The noises propagating through the internal combustion engine in the form of solid-borne sound can be muffled quite well and are thus usually no problem as far as protection against noise is concerned.
The noises traveling through the exhaust system of the internal combustion engine together with the combusted fuel-air mixture in the form of airborne sound are reduced by exhaust mufflers located ahead of the exhaust system discharge opening (tailpipe) and downstream of catalytic converters, if present. Respective mufflers may for instance work according to the absorption and/or reflection principle. The disadvantage of both operating principles is that they require a comparatively large volume and create a comparatively high resistance to the combusted fuel-air mixture, resulting in a drop of the overall efficiency of the vehicle and in increased fuel consumption.
For quite some time, so-called anti-noise systems have been developed as an alternative or supplement to mufflers. Anti-noise systems superimpose electro-acoustically generated anti-noise on airborne noise generated by the internal combustion engine and propagated through the exhaust system. Respective anti-noise systems typically use a so-called Filtered-X, Least Mean Squares (FxLMS) algorithm trying to bring the airborne noise propagating through the exhaust system down to zero (in the case of noise-cancellation) or to a preset threshold (in the case of influencing noise) by outputting sound using at least one loudspeaker. The loudspeaker is usually in fluid communication with the exhaust system. For achieving a completely destructive interference between the sound waves of the airborne sound propagating through the exhaust system and the anti-noise generated by the loudspeaker, the sound waves originating from the loudspeaker have to match the sound waves propagating through the exhaust system in amplitude and frequency with a relative phase shift of 180 degrees. If the sound waves of the airborne noise propagating through the exhaust system match the anti-noise sound waves generated at the loudspeaker in frequency and have a phase shift of 180 degrees relative thereto, but do not match in amplitude, only an attenuation of the sound waves of the airborne sound propagating through the exhaust system results. The anti-noise is calculated separately for each frequency band of the airborne noise propagating through the exhaust pipe using the FxLMS-algorithm by determining a proper frequency and phasing of two sine oscillations being shifted with respect to each other by 90 degrees, and by calculating the required amplitudes for these sine oscillations. The objective of anti-noise systems is that the cancellation or influencing of sound is audible and measurable at least outside of the exhaust system. As the case may be, the cancellation or influencing of sound is audible and measurable also inside the exhaust system.
A system for influencing sound waves propagating through an exhaust system of a vehicle driven by an internal combustion engine will be described below with reference to FIGS. 1A and 1B.
A system's sound generator 3 shown in the schematic perspective view of FIG. 1A comprises a solid two-part enclosure formed by an upper shell 32 and a lower shell 33 which are joined together in an air-tight manner. The enclosure houses an electrodynamic loudspeaker 2 and is connected to an exhaust system 4 by a Y-pipe 1. The Y-pipe 1 comprises a discharge opening 5 at the base of the “Y” for discharging exhaust gases flowing through the exhaust system 4 and sound generated by the loudspeaker 2. By having the connection implemented with the Y-pipe, the thermal stress of the loudspeaker 2 disposed within the sound generator 3 that is caused by the exhaust gases flowing through the exhaust system 4 is kept low. This is required because conventional loudspeakers are configured for an operation within a range of up to a maximum of 200° C. only, while the temperature of the exhaust gases flowing through the exhaust system 4 may be up to between 400° C. and 700° C. A pressure equalizing valve 36 is disposed on the upper shell 32 of the enclosure. The pressure equalizing valve 36 ensures that a pressure inside the enclosure corresponds approximately to a pressure outside of the enclosure. In order to protect the valve 36 disposed on the surface of the upper shell 32 against damage, the upper shell 32 further holds a cast metal ring 37 surrounding the valve 36. The ring 37 has a slot at its bottom for enabling a liquid to drain off from the region surrounded by the ring 37. Finally, the upper shell 32 holds a cable bushing 34 through which connecting wires are fed-through into the inside of the sound generator 3.
FIG. 1B shows a schematic cross section through the sound generator 3 of FIG. 1A. As can be seen, the loudspeaker 2 is a voice coil type loudspeaker 2 comprising a permanent magnet 21, and a funnel-like membrane 22 which are together supported by a loudspeaker basket 23. Hereby, the membrane 22 is connected at its radial outside to the loudspeaker basket 23 by an elastic surround (not shown) and comprises at its radial inside a voice coil (not shown) that moves in bores formed in the permanent magnet 21. By applying an alternating current to the voice coil, force is exerted onto the membrane 22 by the voice coil due to a Lorentz force, causing the membrane 22 to oscillate. The control signals required for operating the voice coil are supplied through the cable bushing 34 disposed on the upper shell 32 of the enclosure by wires 35. The loudspeaker basket 23 is at its radial outside supported by a bell mouth 42 that is connected to the Y-pipe 1 via a connecting pipe 41. The use of bell mouth 42 is required, since the area of the loudspeaker's 2 membrane 22 is larger than the cross-sectional area of the exhaust system 4 in the sound coupling region. The large area of the membrane 22 is necessary to achieve the required sound energy flux. The funnel-like membrane 22 defines an axis of symmetry S forming an angle of 33° with the bottom of the bell mouth 42. The axis of symmetry S is perpendicular to a parting plane 31 along which the enclosure's upper shell 32 and lower shell 33 are joined together. The elastic surround of the loudspeaker 2 is as a matter of fact located in said parting plane 31.
A detriment of the above structure is its sizable overall volume. Due to numerous restrictive installation space conditions in the undercarriage of a vehicle as well as in a vehicle's engine compartment housing the intake system, a corresponding mounting space is only available to a limited extent. Since systems for influencing the sound waves propagating through an exhaust system of a vehicle driven by an internal combustion engine require considerable sound energy fluxes, it is not possible to just reduce the diameter of the loudspeaker. The area of the membrane is instead required to be equal to or larger than the cross sectional area of the exhaust system or the intake system in the sound coupling region. This in turn requires use of a bell mouth at the junction between the membrane of the loudspeaker and the fitting connecting to the exhaust system or intake system.
With the above construction, mounting an above structure proved further to be laborious with the cable bushing and the fed-through wires being frequently damaged. Finally, the pressure equalizing valve having the ring surrounding is expensive to produce because of the numerous individual operations required for its manufacture.